Over-voltage protection of gallium nitride semiconductor devices

ABSTRACT

A monolithically integrated semiconductor assembly is presented. The semiconductor assembly includes a substrate including silicon carbide (SiC), and gallium nitride (GaN) semiconductor device is fabricated on the substrate. The semiconductor assembly further includes at least one transient voltage suppressor (TVS) structure fabricated in or on the substrate, wherein the TVS structure is in electrical contact with the GaN semiconductor device. The TVS structure is configured to operate in a punch-through mode, an avalanche mode, or combinations thereof, when an applied voltage across the GaN semiconductor device is greater than a threshold voltage. Methods of making a monolithically integrated semiconductor assembly are also presented.

BACKGROUND OF THE INVENTION

The invention relates generally to over-voltage protection of gallium nitride (GaN)-based semiconductor devices, and, more particularly, to over-voltage protection of GaN-based transistors.

GaN semiconductor devices, such as, field effect transistors (FETs), in particular, high electron mobility field effect transistors (HEMTs) have found applications across industries, such as, medical, defense, aviation etc. However, GaN devices are susceptible to electrical overstresses due to transient events in the circuit and electro-static discharge (ESD). The electrical stresses can induce avalanching in the device that can lead to device degradation and eventually catastrophic failure. While GaN switches have several advantages, this safety consideration (due to a lack of sustainable avalanching) has precluded their widespread deployment in high speed switching and power electronic systems.

GaN-based devices, in particular transistors, have been unable to demonstrate sustained avalanche due to defects in the crystal. Defects arising from the growth of GaN materials on foreign substrates such as silicon, sapphire, silicon carbide (SiC) or other materials are observed in high densities (greater than 1000 per square centimeter), leading to the GaN material's inability to maintain a stable avalanche condition, leading the GaN material to degrade physically, and irreversibly.

A transient voltage suppressor (TVS) device, more popularly known as a surge protector, is an electronic component that is utilized for protecting sensitive electronics from damage such as voltage spikes. A transient or excess voltage (or current) is a momentary or fleeting surge in the voltage (or current) that may harm the sensitive electronic circuitry.

Silicon based TVS devices have conventionally been used for protecting sensitive electronic components from current or voltage transients caused by lightning strikes or electromagnetic interferences. However, silicon-based TVS devices are more prone to generate high leakage currents as the temperatures increase. Particularly, when the ambient temperature reaches unacceptably high values, for example 225 degrees Celsius, silicon based TVS devices may become unsuitable for operation due to excessive leakage currents.

Accordingly, there is a need for overvoltage protection of GaN devices to preclude avalanche conditions. Further, it may be desirable to provide overvoltage protection of GaN devices during high temperature operation (greater than 150 degrees Celsius).

BRIEF DESCRIPTION OF THE INVENTION

One aspect of the present technique is directed to a monolithically integrated semiconductor assembly. The semiconductor assembly includes a substrate comprising SiC, and a GaN semiconductor device fabricated on the substrate. The semiconductor assembly further includes at least one transient voltage suppressor (TVS) structure fabricated in or on the substrate, where the TVS structure is in electrical contact with the GaN semiconductor device. The TVS structure is configured to operate in a punch-through mode, an avalanche mode, or combinations thereof, when an applied voltage across the GaN semiconductor device is greater than a threshold voltage.

Another aspect of the present technique is directed to a monolithically integrated semiconductor assembly. The semiconductor assembly includes a substrate comprising SiC, and a GaN semiconductor device fabricated on the substrate. The semiconductor assembly further comprises at least one TVS structure comprising SiC fabricated in or on the substrate. The TVS structure is in electrical contact with the GaN semiconductor device, and the TVS structure is configured to operate in a punch-through mode, when an applied voltage across the GaN semiconductor device is greater than a threshold voltage.

Another aspect of the present technique is directed to a method of making a monolithically integrated semiconductor assembly. The method includes (a) providing a substrate comprising SiC; (b) fabricating a GaN semiconductor device on the substrate; (c) fabricating at least one TVS in or on the substrate; and (d) electrically coupling the TVS structure with the GaN semiconductor device. The TVS structure is configured to operate in a punch-through mode, an avalanche mode, or combinations thereof, when an applied voltage across the GaN semiconductor device is greater than a threshold voltage.

DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, in which like characters represent like parts throughout the drawings, wherein:

FIG. 1 is a cross-sectional view of a semiconductor assembly, in accordance with certain embodiments of the invention.

FIG. 2 is a cross-sectional view of a semiconductor assembly, in accordance with certain embodiments of the invention.

FIG. 3 is a cross-sectional view of a semiconductor assembly, in accordance with certain embodiments of the invention.

FIG. 4 is a cross-sectional view of a semiconductor assembly, in accordance with certain embodiments of the invention.

FIG. 5 is a cross-sectional view of a semiconductor assembly, in accordance with certain embodiments of the invention.

FIG. 6 is a cross-sectional view of a semiconductor assembly, in accordance with certain embodiments of the invention.

FIG. 7 is a cross-sectional view of a semiconductor assembly, in accordance with certain embodiments of the invention.

FIG. 8 is a cross-sectional view of a semiconductor assembly, in accordance with certain embodiments of the invention.

DETAILED DESCRIPTION

As discussed in detail below, certain embodiments of the invention include monolithically integrated semiconductor assemblies including a GaN semiconductor device and a TVS structure.

Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, and “substantially” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.

In the following specification and the claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. As used herein, the term “or” is not meant to be exclusive and refers to at least one of the referenced components (for example, a region) being present and includes instances in which a combination of the referenced components may be present, unless the context clearly dictates otherwise.

As used herein, the term “layer” refers to a material disposed on at least a portion of an underlying surface in a continuous or discontinuous manner. Further, the term “layer” does not necessarily mean a uniform thickness of the disposed material, and the disposed material may have a uniform or a variable thickness. Furthermore, the term “a layer” as used herein refers to a single layer or a plurality of layers, unless the context clearly dictates otherwise.

As used herein, the term “disposed on” refers to layers disposed directly in contact with each other or indirectly by having intervening layers there between, unless otherwise specifically indicated. The term “adjacent” as used herein means that the two layers are disposed contiguously and are in direct contact with each other.

In the present disclosure, when a layer/device is being described as “on” another layer or substrate, it is to be understood that the layers/devices can either be directly contacting each other or have one (or more) layer or feature between the layers and devices. Further, the term “on” describes the relative position of the layers/devices to each other and does not necessarily mean “on top of” since the relative position above or below depends upon the orientation of the device to the viewer. Moreover, the use of “top,” “bottom,” “above,” “below,” and variations of these terms is made for convenience, and does not require any particular orientation of the components unless otherwise stated.

As described in detail later, a monolithically integrated semiconductor assembly is presented. The term “monolithically integrated” as used herein refers to a semiconductor assembly having all components manufactured/fabricated into or on top of a single substrate. FIGS. 1 and 2 schematically represent a monolithically integrated semiconductor assembly 100, in accordance with some embodiments of the invention. As illustrated in FIGS. 1 and 2, the semiconductor assembly 100 includes a substrate 110 and a gallium nitride (GaN) semiconductor device 120 is fabricated on the substrate 110. The semiconductor assembly further includes at least one transient voltage suppressor (TVS) structure 130. The TVS structure may be fabricated in the substrate, as illustrated in FIG. 1, or, alternatively, the TVS structure may be fabricated on the substrate, as illustrated in FIG. 2. The TVS structure 130 is in electrical contact 140 with the GaN semiconductor device 120, as indicated in FIGS. 1 and 2.

For the sake of brevity, the terms “gallium nitride semiconductor device” and “GaN device” are used herein interchangeably. Further, the terms “transient voltage suppressor structure” and “TVS structure” are used herein interchangeably.

For particular configurations, the substrate comprises SiC. As noted earlier, embodiments of the invention provide for transient voltage protection of GaN devices at substantially high temperatures, for example, temperatures greater than about 150 degrees Celsius, and more particularly for temperatures greater than about 200 degrees Celsius. Without being bound by any theory, it is believed that a SiC substrate allows for high temperature operation of the GaN device (greater than 150 degrees Celsius).

The semiconductor assembly may include any suitable GaN device 120 based on the end-use application of the assembly. For particular configurations, the GaN device 120 includes a GaN transistor, a GaN diode, or combinations thereof. Suitable non-limiting examples of a GaN transistor include a field effect transistor (FET), such as, for example, a high electron mobility transistor (HEMT), a junction gate field effect transistor (JFET), a metal-oxide-semiconductor field-effect transistor (MOSFET), or combinations thereof. One or more of the GaN devices 120 may be fabricated on the substrate 110 in a lateral configuration or a vertical configuration. For particular configurations, the GaN device 120 is a HEMT.

Referring now to FIG. 3, a schematic view of an exemplary GaN HEMT device 120 is illustrated. The exemplary HEMT device 120, as illustrated in FIG. 3, includes a buffer layer 121 (optional) disposed on the SiC substrate 110. For particular configurations, one or more intervening layers (not shown) may be disposed between the buffer layer 121 and the SiC substrate. For the configuration shown in FIG. 3, a GaN layer 122 is disposed on the buffer layer 121, and an aluminum gallium nitride (AlGaN) layer is disposed on the GaN layer 122. For the illustrated arrangement, the HEMT device 120 further includes a gate terminal 125, a source terminal 126, and a drain 127 terminal, as indicated in FIG. 3. For particular configurations, the GaN layer 122 and the AlGaN layer 123 are intrinsic-type (i-type) and an additional p-type AlGaN layer (not shown) may be disposed between the AlGaN layer 123 and the gate terminal 126. A capping layer 124 (optional) may be further interposed between the source terminal 125/drain terminal 127 and the barrier layer 123.

The GaN HEMT device may be fabricated on the SiC substrate 110 by sequentially disposing or forming one or more of the aforementioned layers. The terms “fabricated on” or “fabricating on” as used herein means that the GaN device 120/TVS structure 130 may be fabricated directly in contact with at least a portion of the substrate 110, or, alternatively, one or more layers/features may be interposed between the GaN devices 120/TVS structure 130 and the substrate 110. For particular configurations, the GaN device 120 may be fabricated on one or more intervening layers or features, which are further disposed on at least a portion of the substrate 110. For example, in some embodiments, the GaN device 120 may be fabricated on at least a portion of a TVS structure 130 disposed on the substrate, as illustrated in FIG. 8 (described in detail later).

As noted earlier, a transient voltage suppressor (TVS) structure, also referred to as a surge protector, is an electronic component that is utilized for protecting sensitive electronics from damage such as voltage spikes. A transient or excess voltage (or current) is a momentary or fleeting surge in the voltage (or current) that may harm sensitive electronic circuitry. Hereinafter, the terms “voltage surge” and “transient voltage” may be used interchangeably to indicate an unexpected or excessive increase in voltage compared to an average voltage across the system. Similarly, the terms “current surge” and “transient current” may be used interchangeably to indicate an unexpected increase in the current compared to an average current transmitted through the system. Hereinafter, the term “transients” may be used to generically refer to transient voltage or transient current.

In general, a transient voltage suppressor device operates on two principles: attenuating excess current or transient current thereby limiting residual current, or diverting a transient or an excess current from the sensitive electronic components. Attenuating a transient current is typically achieved by ensuring that the transient current does not reach or impact the sensitive electronic components, often by using filters inserted in series with the electronic components. Diverting a transient current is typically achieved by using a voltage clamping device or a crowbar type device. In operation, the voltage clamping device has variable impedance that varies in response to the current flowing through the voltage clamping device.

For particular configurations, the TVS structure 130 may be a clamping device. More particularly, the TVS structure 130 may be configured to operate in a punch-through mode, an avalanche mode, or combinations thereof, when an applied voltage across the GaN device 120 is greater than a threshold voltage.

The term “punch-through mode” as used herein means that TVS structure operates using “punch-through,” or also known as, “reach-through” physics, such that, as the voltage across the TVS structure is increased, a depletion region extends across the structure, and large amounts of current are able to flow through the TVS structure. The TVS structure is further able to maintain this condition with minimal change in the voltage across it.

The term “avalanche mode” as used herein refers to the process in which semiconductors sustain an electrical field internal to a region or regions of the device, whereby the process of ionization starts to occur, leading to carrier multiplication. The TVS structures may maintain this condition if so desired, however, this typically involves a non-linear increase in current, above the turn-on voltage of the device, and may see large increases in current and therefore heat in the device in these conditions. The term threshold voltage as used herein refers to the voltage at which the TVS device will effectively turn on, or begin to conduct current.

The TVS structure 130 may comprise silicon carbide, gallium nitride, diamond, aluminum nitride, boron nitride, zinc oxide, gallium oxide, or combinations thereof. For particular configurations, where the TVS structure 130 is fabricated in the SiC substrate, the TVS structure may comprise the same material as the substrate (SiC), as illustrated in FIGS. 3-6. In some such instances, the GaN device 120 may be fabricated on the SiC substrate 110 from regrown epitaxy.

For other configurations, the TVS structure may be fabricated on the substrate, as illustrated in FIGS. 7 and 8. In some such arrangements, the TVS structure 130 may comprise silicon carbide, gallium nitride, or combinations thereof. In such instances, the TVS structure may be fabricated from regrown epitaxy of the same material as the substrate (i.e., SiC, FIG. 8) or from the regrown epitaxy of GaN (FIG. 7).

The TVS structure 130 may be configured laterally, or, alternatively, vertically in the semiconductor assembly 100. FIGS. 3-7 illustrate exemplary configurations in which the TVS structure 130 is configured laterally. FIG. 8 illustrates an exemplary configuration in which the TVS structure 130 is configured vertically.

Referring again to FIGS. 3-8, an exemplary TVS structure 130 includes a first semiconductor region 131 having a first conductivity type, and a second semiconductor region 132 having a second conductivity type and in electrical contact with the first semiconductor region. The TVS structure 130 further includes a third semiconductor region 133 having a first conductivity type, and in electrical contact with the second semiconductor region 132.

For particular configurations, the first conductivity is p-type and the second conductivity is n-type. In such instances, the TVS structure comprises a p-n-p device. For other arrangements, the first conductivity is n-type and the second conductivity is n-type. In such instances, the TVS structure comprises an n-p-n device.

It should be noted herein that the conductivity of a semiconductor material is indicative of the majority and minority charge carriers in the semiconductor material. For example, an n-type semiconductor material includes “negative charge carriers” as majority charge carriers and “positive charge carriers” as minority charge carriers. For example, a p-type semiconductor material includes “negative charge carriers” as minority charge carriers and “positive charge carriers” as majority charge carriers. As is understood by one of ordinary skilled in the art, a “negative charge carrier” refers to electrons whereas a “positive charge carriers” refers to holes.

For particular configurations, the first conductivity type is n+-type and the second conductivity type is p−-type. In an n-p-n type device, when the device is subjected to a potential across the two n-type layers, a depletion layer is formed (mostly) in the p-type layer because its doping is much lower compared to the two n-type layers. For example, one to five orders of magnitude lower, or one-tenth to one ten-thousandth of the dopant concentration of the n-type layers. For a further example, if the doping concentration in the n-type layers is about 10¹⁸/cm³, the doping concentration in the p-type layers would be about 10¹⁵/cm³.

As the voltage across the device is increased, the depletion region extends all across the p-type layer and touches the n-type layer on the other side. This leads to the mode known as “punch-through” as discussed earlier, and a large amount of current begins flowing in the device. The device is able to maintain this condition with minimal change in the voltage across it. A similar explanation describes the operation mode when the polarity of the layers is changed to p-n-p. The avalanche structure may be similar to the punch through structure illustrated in FIGS. 3-8. By adjusting the thickness and doping of the second semiconductor region 132, the TVS structure 130 may be made to work in the avalanche mode instead of punch through mode.

As noted earlier, the TVS structure 130 is in electrical contact 140 with the GaN device 120. For particular configurations, the TVS structure 130 is in electrical contact with the GaN device 120 via a gate-source terminal, a drain-source terminal, a gate-drain terminal, or combinations thereof. FIG. 4 illustrates an arrangement in which the first semiconductor region 131 is in electrical contact 140 with a source terminal 126 of the GaN device 130, and the third semiconductor region 133 is in electrical contact with a gate terminal 125 of the GaN device 120. FIG. 5 illustrates an alternate arrangement in which the first semiconductor region 131 is in electrical contact 140 with a source terminal 126 of the GaN device 130, and the third semiconductor region 133 is in electrical contact with a drain terminal 127 of the GaN device 120.

Further, in some configurations, the semiconductor assembly may include multiple TVS structures 130. FIG. 6 illustrates an arrangement including two TVS structures 130. In such instances, the two TVS structures may be electrically connected to the same terminals the GaN device 120 or to different terminals. FIG. 6 illustrates a configuration in which a first TVS structure 130 is electrically connected to the source terminal 126 and gate terminal 125 of the GaN device 120. The second TVS structure is electrically connected to the source terminal 126 and the drain terminal 127. For particular configurations, the GaN device 120 may be surrounded by a series of rings, including the TVS structure 130, to accommodate larger currents.

For particular configurations, the first semiconductor region 131, the second semiconductor region 132, and the third semiconductor region 133 are formed in the SiC substrate 110, as illustrated in FIGS. 4-6. In such instances, the TVS structure 130 comprises SiC. In such instances, the first semiconductor region 131, the second semiconductor region 132, and the third semiconductor region 133 may be formed in the SiC substrate 110 using any suitable technique, for example, ion implantation of n-type dopants or p-type dopants. Suitable non-limiting examples of p-type dopants include boron, aluminum, gallium, magnesium, carbon, calcium, or combinations thereof. Suitable non-limiting examples of n-type dopants include nitrogen, phosphorus, arsenic, antimony, or combinations thereof.

FIG. 7 illustrates an arrangement, in which the TVS structure 130 is fabricated on the SiC substrate 110. In this exemplary embodiment, the TVS structure 130 comprises GaN. In FIG. 7, the TVS structure 130 further includes a GaN layer 134 disposed on at least a portion of the SiC substrate 110. The first semiconductor region 131, the second semiconductor region 132, and the third semiconductor region 133 are formed in the GaN layer 134, as illustrated in FIG. 7. In such instances, the first semiconductor region 131, the second semiconductor region 132, and the third semiconductor region 133 may be formed in the GaN layer 134 using any suitable technique, for example, ion implantation of n-type dopants or p-type dopants.

For other configurations, the first semiconductor region 131, the second semiconductor region 132, and the third semiconductor region 133 are epitaxially grown on the SiC substrate 110, as illustrated in FIG. 8, which illustrates a vertical configuration of the TVS structure 130. In an exemplary arrangement, the semiconductor assembly 100 includes a mesa structure that includes a SiC substrate (e.g., having an n+-type conductivity). The semiconductor assembly 100 further includes an epitaxially grown first semiconductor region 131 (e.g., having an n+-type conductivity), an epitaxially grown second semiconductor region 132 (e.g., having an p−-type conductivity) in electrical contact with the first semiconductor region 131, and an epitaxially grown third semiconductor region 133 (e.g., having an n+-type conductivity) in electrical contact with the second semiconductor region 132.

In the exemplary arrangement, the second semiconductor region 132 is relatively lightly doped relative to the first semiconductor region 131 and the third semiconductor region 133. For particular configurations, a uniform doping concentration of substrate 110 and regions, 131, 132, and 133 improves a uniformity of the electric field distribution in the depletion region, thereby improving the breakdown voltage characteristic.

Moreover, for particular configurations, as illustrated in FIG. 8, the TVS structure may have a beveled sidewall. The beveled sidewall may be angled about five degrees to about eighty degrees with respect to an interface between adjacent contacting layers to reduce the maximum electric field profile at a surface of the assembly.

For particular configurations, the semiconductor assembly 100 is configured to operate at temperatures greater than about 150 degrees Celsius, more particularly at a temperature greater than about 200 degrees Celsius, and still more particularly at a temperature greater than about 250 degrees Celsius. Further, the semiconductor assembly 100 may be configured to have a leakage current of less than about 1 μA/cm² at 90% of breakdown voltage, and more particularly less than about 0.5 μA/cm² at 90% of breakdown voltage.

The semiconductor assembly 100 may be further configured to have an operation current density greater than about 150 A/cm², and more particularly greater than about 200 A/cm². In some embodiments, the semiconductor assembly 100 is configured to exhibit punch through characteristics between about 5 Volts to about 75 Volts, and more particularly from about 75 Volts to about 200 Volts. The TVS structure 130 may be used in concert with SiC or GaN integrated circuits, in some embodiments.

A method of making a monolithically integrated semiconductor assembly is also presented. Referring again to FIGS. 1 and 2, the method includes the steps of (c) providing a substrate 110 comprising silicon carbide (SiC); (b) fabricating a gallium nitride (GaN) semiconductor device 120 on the substrate 110; (c) fabricating at least one transient voltage suppressor (TVS) 130 in or on the substrate 110; and (d) electrically coupling the TVS structure 130 with the GaN semiconductor device 120. As noted earlier, the TVS structure 130 is configured to operate in a punch-through mode, an avalanche mode, or combinations thereof, when an applied voltage across the GaN semiconductor device 120 is greater than a threshold voltage. It should be noted that the step (b) may be effected before step (c) in some embodiments, or, alternately, step (c) may be performed before (b) in some other embodiments.

The GaN device 120 may be fabricated on the SiC substrate 110 by epitaxially growing one or more layers of the GaN device 120. For example, referring again to FIG. 3, in some embodiments, the GaN device 120 may be fabricated by sequentially disposing or forming one or more of the layers 121, 122, and 123, and forming the source, gate, and drain terminals 125, 126, and 127.

For particular configurations, the method further includes fabricating the TVS structure 130 by forming a first semiconductor region 131 of a first conductivity type, as illustrated in FIGS. 4-8. The method further includes forming a second semiconductor region 133 of a second conductivity type, and in electrical contact with the first semiconductor region 131; and forming a third semiconductor region 133 of a first conductivity type, and in electrical contact with the second semiconductor region 132, as illustrated in FIGS. 4-8.

For particular configurations, the first conductivity is p-type and the second conductivity is n-type. In such instances, the TVS structure includes a p-n-p device. In some other embodiments, the first conductivity is n-type and the second conductivity is n-type. In such instances, the TVS structure includes an n-p-n device. For particular configurations, the first conductivity type is n+-type and the second conductivity type is p−-type.

With continued reference to FIGS. 4-6, for particular processes, the step of forming the TVS structure may include forming the first semiconductor region 131, the second semiconductor region 132, and the third semiconductor region 133 in the SiC substrate 130. In such instances, the TVS structure 130 may be formed using any suitable technique, such as, for example, ion implantation.

FIG. 7 illustrates another particular process, in which the step of forming the TVS structure 130 includes disposing a GaN layer 134 on at least a portion of the SiC substrate 110. The method further includes forming the first semiconductor region 131, the second semiconductor region 132, and the third semiconductor region 133 in the GaN layer 134 to form the TVS structure 130, as illustrated in FIG. 7. In such instances, the TVS structure 130 may be formed using any suitable technique, such as, for example, ion implantation.

For other particular processes, as illustrated in FIG. 8, the step of forming the TVS structure 130 may include epitaxially growing the first semiconductor region 131, the second semiconductor region 132, and the third semiconductor region 133 on the SiC substrate 110. As illustrated in FIG. 8, in such processes, the GaN device 120 is fabricated on the TVS structure 130 after the step of TVS fabrication on the SiC substrate 110.

The method may further include electrically coupling 140 the TVS structure 130 with a GaN semiconductor device 120 via a gate-source terminal, a drain-source terminal, a gate-drain terminal, or combinations thereof, as illustrated in FIGS. 4-7.

The above-described semiconductor assemblies provide a cost-effective and reliable means for overvoltage protection of GaN devices to preclude avalanche conditions. More specifically, the semiconductor assemblies described herein provide for overvoltage protection of GaN devices during high temperature operation (greater than 150 degrees Celsius).

The appended claims are intended to claim the invention as broadly as it has been conceived and the examples herein presented are illustrative of selected embodiments from a manifold of all possible embodiments. Accordingly, it is the Applicants' intention that the appended claims are not to be limited by the choice of examples utilized to illustrate features of the present invention. As used in the claims, the word “comprises” and its grammatical variants logically also subtend and include phrases of varying and differing extent such as for example, but not limited thereto, “consisting essentially of” and “consisting of.” Where necessary, ranges have been supplied; those ranges are inclusive of all sub-ranges there between. It is to be expected that variations in these ranges will suggest themselves to a practitioner having ordinary skill in the art and where not already dedicated to the public, those variations should where possible be construed to be covered by the appended claims. It is also anticipated that advances in science and technology will make equivalents and substitutions possible that are not now contemplated by reason of the imprecision of language and these variations should also be construed where possible to be covered by the appended claims. 

1. A monolithically integrated semiconductor assembly, comprising: a substrate comprising silicon carbide (SiC); a gallium nitride (GaN) semiconductor device fabricated on the substrate; and at least one transient voltage suppressor (TVS) structure fabricated in or on the substrate, wherein the TVS structure is in electrical contact with the GaN semiconductor device, and wherein the TVS structure is configured to operate in a punch-through mode, an avalanche mode, or combinations thereof, when an applied voltage across the GaN semiconductor device is greater than a threshold voltage.
 2. The semiconductor assembly of claim 1, wherein the TVS structure comprises at least one of silicon carbide, gallium nitride, diamond, aluminum nitride, boron nitride, or combinations thereof.
 3. The semiconductor assembly of claim 1, wherein the TVS structure is configured laterally.
 4. The semiconductor assembly of claim 1, wherein the TVS structure is configured vertically.
 5. The semiconductor assembly of claim 1, wherein the TVS structure comprises: a first semiconductor region having a first conductivity type; a second semiconductor region having a second conductivity type, and in electrical contact with the first semiconductor region; and a third semiconductor region having a first conductivity type, and in electrical contact with the second semiconductor region.
 6. The semiconductor assembly of claim 5, wherein the first conductivity type is n+-type and the second conductivity type is p-type.
 7. The semiconductor assembly of claim 5, wherein the first semiconductor region, the second semiconductor region, and the third semiconductor region are formed in the SiC substrate.
 8. The semiconductor assembly of claim 5, wherein the first semiconductor region, the second semiconductor region, and the third semiconductor region are epitaxially grown on the SiC substrate.
 9. The semiconductor assembly of claim 5, wherein the TVS structure further comprises a GaN layer disposed on at least a portion of the SiC substrate, and wherein the first semiconductor region, the second semiconductor region, and the third semiconductor region are formed in the GaN layer.
 10. The semiconductor assembly of claim 1, wherein the TVS structure is in electrical contact with the GaN semiconductor device via a gate-source terminal, a drain-source terminal, a gate-drain terminal, or combinations thereof.
 11. The semiconductor assembly of claim 1, wherein the GaN semiconductor device comprises a high electron mobility transistor (HEMT), a junction gate field effect transistor (JFET), a metal-oxide-semiconductor field-effect transistor (MOSFET), a diode, or combinations thereof.
 12. A monolithically integrated semiconductor assembly, comprising: a substrate comprising silicon carbide (SiC); a gallium nitride (GaN) semiconductor device fabricated on the substrate; and at least one transient voltage suppressor (TVS) structure comprising silicon carbide (SiC) fabricated in or on the substrate, wherein the TVS structure is in electrical contact with the GaN semiconductor device, and wherein the TVS structure is configured to operate in a punch-through mode when an applied voltage across the GaN semiconductor device is greater than a threshold voltage.
 13. The monolithically integrated assembly of claim 12, wherein the assembly is configured to operate at temperatures greater than 150 degree Celsius.
 14. The monolithically integrated assembly of claim 12, wherein the assembly is configured to have a leakage current of less than about 1 μA/cm² at 90% of a breakdown voltage.
 15. The monolithically integrated assembly of claim 12, wherein the assembly is configured to have an operation current density greater than about 150 A/cm².
 16. A method of making a monolithically integrated semiconductor assembly comprising: (a) providing a substrate comprising silicon carbide (SiC); (b) fabricating a gallium nitride (GaN) semiconductor device on the substrate; (c) fabricating at least one transient voltage suppressor (TVS) in or on the substrate; and (d) electrically coupling the TVS structure with the GaN semiconductor device, wherein the TVS structure is configured to operate in a punch-through mode, an avalanche mode, or combinations thereof, when an applied voltage across the GaN semiconductor device is greater than a threshold voltage.
 17. The method of claim 16, wherein the step (c) comprises: forming a first semiconductor region of a first conductivity type; forming a second semiconductor region of a second conductivity type, and in electrical contact with the first semiconductor region; and forming a third semiconductor region of a first conductivity type, and in electrical contact with the second semiconductor region.
 18. The method of claim 17, wherein the first conductivity type is n+-type and the second conductivity type is p-type.
 19. The method of claim 17, wherein the step (c) comprises forming the first semiconductor region, the second semiconductor region, and the third semiconductor region in the SiC substrate by ion implantation.
 20. The method of claim 17, wherein the step (c) comprises epitaxially growing the first semiconductor region, the second semiconductor region, and the third semiconductor region on the SiC substrate.
 21. The method of claim 17, further comprising disposing a GaN layer on at least a portion of the SiC substrate, and forming the first semiconductor region, the second semiconductor region, and the third semiconductor region in the GaN layer.
 22. The method of claim 16, wherein the step (d) comprises electrically coupling the TVS structure with a GaN semiconductor device via a gate-source terminal, a drain-source terminal, a gate-drain terminal, or combinations thereof.
 23. The method of claim 16, wherein the GaN semiconductor device comprises a high electron mobility transistor (HEMT), a junction gate field effect transistor (JFET), a metal-oxide-semiconductor field-effect transistor (MOSFET), a diode, or combinations thereof. 